It varies you have to test using prime95 small cpu test. With your cooler I would think 3.9Ghz at the least. What overclock is stable with 1.35v you just have to test and see. Put it to 3.9Ghz and test and if you get no errors or crashes then go 4Ghz. Test again and if all is well I suggest smaller increases. The 1.35v is about the highest safe voltage and 4.1Ghz probably will not be stable.
On the topic of prime95, I honestly dont know how to use it correctly. Is there a link on youtube you know that would explain it better?